Welded aerospace structure using a hybrid metal webbed composite beam

ABSTRACT

A hybrid metal webbed composite beam includes a metal I or T web section and a composite cap formed over and adhered to the I or T web. The beam incorporates the advantages of metals and composites in modern aerospace construction allowing thermoplastic welding of the beam to skins while having the strength of metal.

TECHNICAL FIELD

The present invention relates to a welded structure using hybridcomposite stiffening element, particularly a titanium I or T section webwith composite caps that are formed into flanges of the web.

BACKGROUND ART

Aircraft are expensive to manufacture because safety concerns dictatequality manufacture while weight limits range, performance, and payload.There are significant design challenges. Large aircraft are commonlymade from aluminum alloys riveted and fastened together. The fastenersadd significantly to the total weight. Military aircraft, especiallyfighters, are more and more being made from thermoset or thermoplasticmaterials for improved strength-to-weight ratios, but the constructionstill parrots metal construction with fasteners to join parts intoassemblies. Military airplanes must satisfy even more requirements thancommercial transport airplanes, such as the Congressionally-imposed"Live Fire" tests. Military airplanes also have unusual performancerequirements. To obtain the desired performance, reducing the cost andweight of construction are two enviable objectives.

The present invention is a hybrid beam that combines metal withcomposites to marry the advantages each offers. This description willfirst discuss the problems of composite manufacturing before turning toa brief discussion of emerging solutions to address the cost and weightobjectives where the beam of the present invention offers the greatestleverage.

1. Composite Manufacturing

Fiber-reinforced organic resin matrix composites have a highstrength-to-weight ratio or a high stiffness-to-weight ratio anddesirable fatigue characteristics that make them increasingly popular asa replacement for metal in aerospace applications where weight,strength, or fatigue is critical. Organic resin composites, be theythermoplastics or thermosets, are expensive today. Improvedmanufacturing processes would reduce touch labor and forming time.

Prepregs combine continuous, woven, or chopped reinforcing fibers withan uncured, matrix resin, and usually comprise fiber sheets with a thinfilm of the resin matrix. Sheets of prepreg generally are placed(laid-up) by hand or with fiber placement machines directly upon a toolor die having a forming surface contoured to the desired shape of thecompleted part or are laid-up in a flat sheet which is then draped andformed over the tool or die to the contour of the tool. Then the resinin the prepreg lay up is consolidated (i.e. pressed to remove any air,gas, or vapor) and cured (i.e., chemically converted to its final formusually through chain-extension) in a vacuum bag process in an autoclave(i.e., a pressure oven) to complete the part.

The tools or dies for composite processing typically are formed to closedimensional tolerances. They are massive, must be heated along with theworkpiece, and must be cooled prior to removing the completed part. Thedelay caused to heat and to cool the mass of the tools addssubstantially to the overall time necessary to fabricate each part.These delays are especially significant when the manufacturing run islow rate where the dies need to be changed frequently, often afterproducing only a few parts of each kind. An autoclave has similarlimitations; it is a batch operation.

In hot press forming, the prepreg is laid-up to create a preform, whichis bagged (if necessary), and placed between matched metal tools thatinclude forming surfaces to define the internal, external, or both moldlines of the completed part. The tools and composite preform are placedwithin a press and then the tools, press, and preform are heated.

The tooling in autoclave or hot press fabrication is a significant heatsink that consumes substantial energy. Furthermore, the tooling takessignificant time to heat the composite material to its consolidationtemperature and, after curing the composite, to cool to a temperature atwhich it is safe to remove the finished composite part.

As described in U.S. Pat. No. 4,657,717, a flat composite prepreg panelwas sandwiched between two metal sheets made from a superplasticallyformable alloy and then formed against a die having a surface preciselycontoured to the final shape of the part.

Attempts have been made to reduce composite fabrication times byactively cooling the tools after forming the composite part. Theseattempts have shortened the time necessary to produce a composite part,but the cycle time for and cost of heating and cooling remainsignificant contributors to overall fabrication costs. Designing andmaking tools to permit their active cooling increases their cost.

Boeing described a process for organic matrix forming and consolidationusing induction heating in U.S. patent application Ser. No. 08/169,655.There, prepregs were laid up in a flat sheet and were sandwiched betweenaluminum susceptor facesheets. The facesheets were susceptible toheating by induction and formed a retort to enclose the prepreg preform.To ensure an inert atmosphere around the composite during curing and topermit withdrawing volatiles and outgassing from around the compositeduring the consolidation, we welded the facesheets around theirperiphery. Such welding unduly impacts the preparation time and the costfor part fabrication. It also ruined the facesheets (i.e., prohibitedtheir reuse which added a significant cost penalty to each partfabricated with this approach). Boeing also described in U.S. patentapplication Ser. No. 08/341,779 a technique that readily and reliablyseals facesheets of the retort without the need for welding and permitsreuse of the facesheets in certain circumstances. Our "bag-and-seal"technique applies to both resin composite and metal processing.

2. Processing in an Induction Press

The dies or tooling for induction processing are cast ceramic because aceramic is not susceptible to induction heating and, preferably, is athermal insulator (i.e., a relatively poor conductor of heat). Castceramic tooling is strengthened and reinforced internally, withfiberglass rods or other appropriate reinforcements and externally withmetal or other durable strongbacks to permit it to withstand thetemperatures and pressures necessary to form, to consolidate, orotherwise to process the composite materials or metals. Cast ceramictools cost less to fabricate than metal tools of comparable size andhave less thermal mass than metal tooling, because they are unaffectedby the induction field. Because the ceramic tooling is not susceptibleto induction heating, it is possible to embed induction heating elementsin the ceramic tooling and to heat the composite or metal retort withoutsignificantly heating the tools. The induction heating elementsthemselves connect to form a water cooling network. Thus, inductionheating can reduce the time required and energy consumed to fabricate apart.

While graphite or boron fibers can be heated directly by induction, mostorganic matrix composites require a susceptor in or adjacent to thecomposite material preform to achieve the necessary heating forconsolidation or forming. The susceptor is heated inductively andtransfers its heat principally through conduction to the preform orworkpiece that, in our prior work, is sealed within the susceptorretort. Enclosed in the metal retort, the workpiece does not experiencethe oscillating magnetic field which instead is absorbed in the retortsheets. Heating is by conduction from the retort to the workpiece.

Induction focuses heating on the retort (and workpiece) and eliminateswasteful, inefficient heat sinks. Because the ceramic tools in ourinduction heating workcell do not heat to as high a temperature as themetal tooling of conventional, prior art presses, problems caused bydifferent coefficients of thermal expansion between the tools and theworkpiece are reduced. Furthermore, we are energy efficient becausesignificantly higher percentages of our input energy go to heating theworkpiece than occurs with conventional presses. Our reduced thermalmass and ability to focus the heating energy permits us to change theoperating temperature rapidly which improves the products we produce.Finally, our shop environment is not heated as significantly from theradiation of the large thermal mass of a conventional press, and is asafer and more pleasant environment for the press operators.

In induction heating for consolidating and/or forming organic matrixcomposite materials, we place a thermoplastic organic matrix compositepreform of PEEK or ULTEM, for example, within a metal susceptor envelope(i.e., retort). These thermoplastics have a low concentration ofresidual volatile solvents and are easy to use. The susceptor facesheetsof the retort are inductively heated to heat the preform. We applyconsolidation and forming pressure to consolidate and, if applicable, toform the preform at its curing temperature. The sealed susceptor sheetsform a pressure zone. We evacuate the pressure zone in the retort in amanner analogous to conventional vacuum bag processes for resinconsolidation, or, for low volatiles resins, like ULTEM, we canpressurize this zone to enhance consolidation. The retort is placed inan induction heating press on the forming surfaces of dies having thedesired shape of the molded composite part. After the retort (andpreform) are inductively heated to the desired elevated temperature, weapply differential pressure (while maintaining the vacuum in thepressure zone around the preform) across the retort which functions as adiaphragm in the press to form the preform against the die into thedesired shape of the completed composite panel.

The retort often includes three susceptor sheets sealed around theirperiphery to define two pressure zones. The first pressure zonesurrounds the composite panel/preform or metal workpiece and isevacuated and maintained under vacuum. The second pressure zone ispressurized (i.e., flooded with gas) at the appropriate time and rate tohelp form the composite panel or workpiece. The shared wall of the threelayer sandwich that defines the two pressure zones acts as a diaphragmin this situation.

We can perform a wide range of manufacturing operations in our inductionheating press. These operations have optimum operating temperaturesranging from about 350° F. (175° C.) to about 1950° F. (1066° C.). Foreach operation, we usually need to hold the temperature relativelyconstant for several minutes to several hours while we complete theoperations. While we can achieve temperature control by controlling theinput power fed to the induction coil, we have discovered a better andsimpler way that capitalizes on the Curie temperature. By judiciousselection of the metal or alloy in the retort's susceptor facesheets, wecan avoid excessive heating irrespective of the input power. Withimproved control and improved temperature uniformity in the workpiece,we produce better products. Our method capitalizes on the Curietemperature phenomenon to control the absolute temperature of theworkpiece and to obtain substantial thermal uniformity in the workpiece,by matching the Curie temperature of the susceptor to the desiredtemperature of the induction heating operation being performed. Thistemperature control method is explained in greater detail in our U.S.patent application Ser. No. 08/469,604 by Hansen et al. entitled Methodfor Achieving Thermal Uniformity in Induction Processing of OrganicMatrix Composites or Metals, which we incorporate by reference.

3. Thermoplastic Welding

Three major joining technologies exist for aerospace compositestructure: mechanical fastening; adhesive bonding; and welding. Bothmechanical fastening and adhesive bonding are costly, time consumingassembly steps that introduce excess cost even if the parts that areassembled are fabricated from components produced by an emerging, costefficient process. Mechanical fastening requires expensive holelocating, drilling, shimming, and fastener installation, while adhesivebonding often requires complicated surface pretreatments.

In contrast, composite welding, which eliminates fasteners, features theability to join thermoplastic are thermoset composite components at highspeeds with minimum touch labor and little, if any, pretreatments. Inour experience, the welding interlayer (comprising the susceptor andsurrounding thermoplastic resin either coating the susceptor orsandwiching it) also can simultaneously take the place of shims requiredin mechanical fastening. As such, composite welding holds promise to bean affordable joining process. For "welding" thermoplastic and thermosetcomposite parts together, the resin that the susceptor melts functionsas a hot melt adhesive. If fully realized, the thermoplastic-thermosetbonding will further reduce the cost of composite assembly. We use theterm "thermoplastic welding" to refer to either bonding operation unlessthe context forces a different meaning.

There is a large stake in developing a successful induction weldingprocess. Its advantages versus traditional composite joining methodsare:

reduced parts count versus fasteners

minimal surface preparation, in most cases a simple solvent wipe toremove surface contaminants

indefinite shelf life at room temperature

short process cycle time, typically measured in minutes

enhanced joint performance, especially hot/wet and fatigue

permits rapid field repair of composites or other structures.

There is little or no loss of bond strength after prolonged exposure toenvironmental influences.

U.S. Pat. No. 4,673,450 describes a method to spot weld graphite fiberreinforced PEEK composites using a pair of electrodes After rougheningthe surfaces of the prefabricated PEEK composites in the region of thebond, Burke placed a PEEK adhesive ply along the bond line, applied apressure of about 50-100 psi through the electrodes, and heated theembedded graphite fibers by applying a voltage in the range of 20-40volts at 30-40 amps for approximately 5-10 seconds with the electrodes.Access to both sides of the assembly is required in this process whichlimits its application.

Prior art disclosing thermoplastic welding with induction heating isillustrated by U.S. Pat. Nos. 3,966,402 and 4,120,712. In these patents,the metallic susceptors used are of a conventional type having a regularpattern of openings of traditional manufacture. Achieving a uniform,controllable temperature in the bondline, which is crucial to preparinga thermoplastic weld of adequate integrity to permit use of welding inaerospace primary structure, is difficult with those conventionalsusceptors, as we discussed and illustrated in our copending U.S. patentapplication Ser. No. 08/068,520.

Thermoplastic welding is a process for forming a fusion bond between twofaying thermoplastic faces of two parts. A fusion bond is created whenthe thermoplastic on the surface of the two thermoplastic compositeparts is heated to the melting or softening point and the two surfacesare brought into contact, so that the molten thermoplastic mixes, andthe surfaces are held in contact while the thermoplastic cools below thesoftening temperature.

Simple as the thermoplastic welding process sounds, and easy as it is toperform in the laboratory on small pieces, it becomes difficult toperform reliably and repeatably in a real factory on full-scale parts tobuild a large structure such as an airplane wing box. The difficulty isin getting the proper amount of heat to the bondline without overheatingthe entire structure, and also in achieving intimate contact of thefaying surfaces of the two parts at the bondline during heating andcooling despite the normal imperfections in the flatness of compositeparts, thermal expansion of the thermoplastic during heating to thesoftening or melting temperature, flow of the thermoplastic out of thebondline under pressure (i.e., squeeze out), and then contraction of thethermoplastic in the bondline during cooling.

The exponential decay of the strength of magnetic fields dictates that,in induction welding processes, the susceptible structure closest to theinduction coil will be the hottest, since it experiences the strongestfield. Therefore, it is difficult to obtain adequate heating at the bondline between two graphite or carbon fiber reinforced resin matrixcomposites relying on the susceptibility of the fibers alone as thesource of heating in the assembly. For the inner plies to be hot enoughto melt the resin, the outer plies closer to the induction coil and inthe stronger magnetic field are too hot. The matrix resin in the entirepiece of composite melts. The overheating results in porosity in theproduct, delamination, and, in some case, destruction or denaturing ofthe resin. To avoid overheating of the outer plies and to insureadequate heating of the inner plies, we use a susceptor of significantlyhigher conductivity than the fibers to peak the heating selectively atthe bondline. An electromagnetic induction coil heats a susceptor tomelt and cure a thermoplastic resin (also sometimes referred to as anadhesive) to bond the elements of the assembly together.

The current density in the susceptor may be higher at the edges of thesusceptor than in the center because of the nonlinearity of the coil,such as occurs when using a cup core induction coil like that describedin U.S. Pat. No. 5,313,037. Overheating the edges of the assembly canresult in underheating the center, either condition leading to inferiorwelds because of non-uniform curing. It is necessary to have an open ormesh pattern in the susceptor embedded at the bondline to allow theresin to create the fusion bond between the composite elements of theassembly when the resin heats and melts.

a. Moving coil welding processes

In U.S. patent application Ser. No. 08/286,360, we described a tailoredsusceptor for approaching the desired temperature uniformity. Thissusceptor, designed for use with the cup coil of U.S. Pat. No.5,313,037, relied upon carefully controlling the geometry of openings inthe susceptor (both their orientation and their spacing) to distributethe heat evenly. We suggested using a regular array of anisotropic,diamond shaped openings with a ratio of the length (L) to the width (W)greater than 1 to provide a superior weld by producing a more uniformtemperature than obtainable using a susceptor having a similar array,but one where the L/W ratio was one. By changing the length to widthratio (the aspect ratio) of the diamond-shaped openings in thesusceptor, we achieved a large difference in the longitudinal andtransverse conductivity in the susceptor, and, thereby, tailored thecurrent density within the susceptor. A tailored susceptor havingopenings with a length (L) to width (W) ratio of 2:1 has a longitudinalconductivity about four times the transverse conductivity. In additionto tailoring the shape of the openings to tailor the susceptor, wealtered the current density in regions near the edges by increasing thefoil density (i.e., the absolute amount of metal). Increasing the foildensity along the edge of the susceptor increases the conductivity alongthe edge and reduces the current density and the edge heating. Weincreased foil density by folding the susceptor to form edge strips ofdouble thickness or by compressing openings near the edge of anotherwise uniform susceptor. We found these susceptors difficult toreproduce reliably. Also, their use forced careful placement andalignment to achieve the desired effect.

The tailored susceptor was designed to use with the cup coil of U.S.Pat. No. 5,313,037, where the magnetic field is strongest near the edgesbecause the central pole creates a null at the center. Therefore, thetailored susceptor was designed to counter the higher field at the edgesby accommodating the induced current near the edges. The highlongitudinal conductivity encouraged induced currents to flowlongitudinally.

Our selvaged susceptor for thermoplastic welding which is described inU.S. patent application Ser. No. 08/314,027 controls the current densitypattern during eddy current heating by an induction coil to providesubstantially uniform heating to a composite assembly and to insure thestrength and integrity of the weld in the completed part. This susceptoris particularly desirable for welding ribs between prior welded sparsusing an asymmetric induction coil (described in U.S. patent applicationSer. No. 08/349,647, which we incorporate by reference), because, withthat coil, it provides a controllable area of intense, uniform heating,a trailing region with essentially no heating, and a leading region withminor preheating.

The power (P) or power density! which the susceptor dissipates as heatfollows the well-known equation for power loss in a resistor: P=(J²)(R)wherein J is the eddy current (or its density) and R is the impedance(i.e., resistance) of any segment of the eddy path. The heating achieveddirectly corresponds to the power (or power density).

We achieve better performance (i.e., more uniform heating) in ribwelding by using a selvaged susceptor having edge strips withoutopenings. The resulting susceptor, then, has a center portion with aregular pattern of opening and solid foil edges, which we refer to asselvage edge strips. We embed the susceptor in a thermoplastic resin tomake a susceptor/resin tape that is easy to handle and to use inperforming the composite pieces prior to welding. Also, we havediscovered that, with a selvaged susceptor, the impedance of the centralportion should be anisotropic with a lower transverse impedance than thelongitudinal impedance. Here, the L/W ratio of diamond shaped openingsshould be less than or equal to one. That is, unlike our tailoredsusceptor of U.S. patent application Ser. No. 08/286,360, L for theselvaged susceptor should be less than W. With this new selvagedsusceptor in the region immediately under the asymmetric induction workcoil, we encourage the current to flow across the susceptor to the edgeswhere the current density is lowest and the conductivity, highest.

Generally, we form the selvaged susceptor somewhat wider than normal sothat the selvage edge strips are not in the bondline. We remove theselvage edge strips after forming the weld, leaving only a perforatedsusceptor foil in the weld. This foil has a relatively high open areafraction.

Significant effort has been expended in developing inductor andsusceptor systems to optimize the heating of the bondline inthermoplastic assemblies. Induction coil structures and tailoredsusceptors have now been developed that provide adequate control anduniformity of heating of the bondline, but a big hurdle remaining toperfecting the process to the point of practical utility for producinglarge scale aerospace-quality structures in a production environment isthe aspect of the process dealing with the control of the surfacecontact of the faying surfaces of the two parts to be welded together,and the timing, intensity, and schedule of heat application so thematerial at the faying surfaces is brought to and maintained within theproper temperature range for the requisite amount of time for anadequate bond to form, and is maintained in intimate contact while themelted or softened material hardens in its bonded condition.

Large scale parts such as wing spars and ribs, and the wing skins thatare bonded to the spars and ribs, are typically on the order of 20-30feet long at present, and potentially can be hundreds of feet in lengthwhen the process is perfected for commercial transport aircraft. Partsof this magnitude are difficult to produce with perfect flatness.Instead, the typical part will have various combinations of surfacedeviations from perfect flatness, including large scale waviness in thedirection of the major length dimension, twist about the longitudinalaxis, dishing or sagging of "I" beam flanges, and small scale surfacedefects such as asperities and depressions. These irregularitiesinterfere with full surface area contact between the faying surfaces ofthe two parts and result in surface contact only at a few "high points"across the intended bondline. Additional surface contact can be achievedby applying pressure to the parts to force the faying surfaces intocontact, but full intimate contact is difficult or impossible to achievein this way. Applying heat to the interface by electrically heating thesusceptor in connection with pressure on the parts flattens theirregularities, but the time that is needed to achieve full intimatecontact with the use of heat and pressure is excessive. The delay needto flatten can result in deformation of the top part. When the overalltemperature of the "I" beam flanges reaches the softening point, theywill begin to yield or sag under the application of the pressure neededto achieve a good bond.

Our multipass thermoplastic welding process described in U.S. patentapplication Ser. No. 08/367,546 enables a moving coil welding process toproduce continuous or nearly continuous fusion bonds over the full areaof the bondline to yield very high strength welds reliably, repeatablyand with consistent quality. This process produces improved low cost,high strength composite assemblies of large scale parts fusion bondedtogether with consistent quality, and uses a schedule of heatapplication that maintains the overall temperature of the structurewithin the limit in which it retains its high strength, so it requiresno internal tooling to support the structure against sagging whichotherwise could occur above the high strength temperature limit. Theprocess also produces nearly complete bondline area fusion on standardproduction composite material parts having the usual surfaceimperfections and deviations from perfect flatness, while eliminatingfasteners and the expense of drilling holes, inspecting the holes andthe fasteners, inspecting the fasteners after installation, sealingbetween the parts and around the fastener and the holes; reducingmismatch of materials; and eliminating arcing from the fasteners.

In the process, an induction heating work coil is passed multiple timesover a bondline while applying pressure in the region of the coil to thecomponents to be welded, and maintaining the pressure until the resinhardens. The resin at the bondline is heated to the softening or meltingtemperature with each pass of the induction work coil and pressure isexerted to flow the softened/melted resin in the bondline and reduce thethickness of the bondline while improving the intimacy of the fayingsurface contact with each pass to militate for complete continuity ofbond. The total time at the softened or melted condition of thethermoplastic in the faying surfaces is sufficient to attain deepinterdiffusion of the polymer chains in the materials of the two fayingsurfaces throughout the entire length and area of the bondline, therebyproducing a bondline of improved strength and integrity in the completedpart, but the total time of the faying surfaces at softened temperatureis in separate time segments which allows time for the heat in theinterface to dissipate without raising the temperature of the entirestructure to the degree at which it loses its strength and begins tosag, so the desired shape and size of the final assembly is maintained.

A structural susceptor allows us to include fiber reinforcement withinthe weld resin to alleviate residual tensile strain otherwise present inan unreinforced weld. The susceptor includes alternating layers of thinfilm thermoplastic resin sheets and fiber reinforcement (usually wovenfiberglass fiber) sandwiching the conventional metal susceptor that isembedded in the resin. While the number of total plies in thisstructural susceptor is usually not critical, we prefer to use at leasttwo plies of fiber reinforcement on each side of the susceptor. Thisstructural susceptor is described in greater detail in our U.S patentapplication Ser. No. 08/471,625 entitled A Structural Susceptor forThermoplastic Welding, which we incorporate by reference.

The structural susceptor permits gap filling between the weldedcomposite laminates which tailors the thickness (number of plies) in thestructural susceptor to fill the gaps, thereby eliminating costlyprofilometry of the faying surfaces and the inherent associated problemof resin depletion at the faying surfaces caused by machining thesurfaces to have complementary contours. Standard manufacturingtolerances produce gaps as large as 0.120 inch, which is too wide tocreate a quality weld using the conventional susceptors.

We can easily tailor the thickness of the structural susceptor to matchthe measured gap by scoring through the appropriate number of plies ofresin and fiber reinforcement and peeling them off. In doing so, a resinrich layer will be on both faying surfaces and this layer should insurebetter performance from the weld.

b. Fixed coil induction welding

We have also experimented with thermoplastic welding using our inductionheating workcell and, of course, discovered that the process differsfrom the moving coil processes because of the coil design and resultingmagnetic field. We believe that our fixed coil workcell presents promisefor welding at faster cycle times than the moving coil processes becausewe can heat multiple susceptors simultaneously. The keys to the process,however, are achieving controllable temperatures at the bondline in areliable and reproducible process that assure quality welds of high bondstrength. Our fixed coil induces currents to flow in the susceptordifferently from the moving coils and covers a larger area.Nevertheless, we have developed processing parameters that permitwelding with our induction heating workcell using a susceptor at thebondline.

To prevent sagging of the prefabricated resin composite parts when theresin melts along the bondline and in the faying surfaces during athermoplastic welding process, we can use inflatable bladders to fillthe hollow spaces in an assembly of the parts to provide a reactionforce. We protect the bladder by adding insulation at the interfaces ofthe parts. The insulation covers sharp edges in the parts and shieldsthe bladder from contact with the hot susceptor that we embed in thebondlines. When we weld, we press a preform assembly of the parts with adie set to have a pressure of about 30-100 psi on each bondline. Weinflate the bladder so that the reaction force from the bladder preventssagging. The typical bladder pressure is about 30-100. Generally, wefill the bladder with air but we might also use a mineral oil or otherfluid.

Susceptors along the bondline are problematic, even if we use areinforced susceptor where the metal foil is woven with the warp threadsso that there is a fiber-resin interface rather than a neat metal foilto which the resin does not wet well. We described such a reinforcedsusceptor in one U.S. patent application Ser. No. 08/469,986 entitled AReinforced Susceptor for Induction of Thermoplastic Composites, which weincorporate by reference. The problems center on the fact that the metalis a foreign material in the weld or bond present only as a heatingsource and detriment in other respects. Therefore, welding would beimproved if the susceptor could be eliminated.

Sagging is also a problem and countering the sagging is an even biggerproblem especially when trying to construct realistic aerospacestructural assemblies of ribs, spars, closeouts, and skins. Suchassemblies usually include isolated regions where it is difficult toinsert or to remove support tooling. Therefore, a design that reducesthe possibility of sagging or that inherently includes support toolingwould simplify the welding process.

SUMMARY OF THE INVENTION

A hybrid metal webbed composite beam used in the welded structure of thepresent invention preferably includes a titanium sine wave spar web withcomposite caps that are formed onto flanges of the web. The compositecaps are adhered to the titanium web with a hot shoe forming tool toform prefabricated and consolidated thermoplastic composite laminatesinto a "C" section onto the titanium web or through the lay up, bagging,and consolidation of flexible prepregs onto the titanium flange as aninternal tool. The use of the hybrib titanium/composite beam reducesseveral problems associated with thermoplastic welding of spars and ribsto skins. The titanium I-beam or sinewave provides an internal tool toprevent deformation of the spar caps or delamination of the capcomposite plies during the welding process.

The hybrid beam is also superior in performance to an all compositespar, rib, or frame in the following areas: cap pulloff, web blastresistance, bending stiffness, dimensional accuracy, and ability to weldinto a composite skinned wing structure. The beam uses both the triaxialstrength and fracture resistance of titanium in the web and tee jointareas, and takes advantage of the high specific stiffness of compositematerials in the outer chord of the caps. The internal titanium Isection functions as an internal tool to allow welding of thethermoplastic caps to thermoplastic skins without additional supporttooling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of our hybrid metal webbedcomposite beam.

FIG. 2 is a schematic perspective view of forming the beam of FIG. 1with a hot shoe.

FIG. 3 is a schematic perspective view of forming the beam of FIG. 1 ina lay up and consolidation process.

FIG. 4 is a perspective view of our induction heating workcell.

FIG. 5 is a schematic cross sectional view of the induction heatingworkcell of FIG. 4.

FIG. 6 is a schematic cross sectional view of our induction heatingworkcell adapted for thermoplastic welding of a wingskin/spar assembly.

FIG. 7 is another schematic cross sectional view of the workcell of FIG.6 rotated 90° from the view in FIG. 6.

FIG. 8 is a schematic cross sectional view of a reinforced susceptor.

FIG. 9 is a schematic sectional view of an alternative beam.

FIG. 10 illustrates schematically in cross section a "smart" susceptorbrazed to the spar cap flange to provide improved pulloff strength.

FIG. 11 illustrates schematically in cross section a double-sided"smart" susceptor similar to the susceptor of FIG. 10.

FIG. 12 illustrates schematically in cross section the susceptor of FIG.11 interfacing at a weld between a composite skin and a composite sparcap.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

1. Our New Hybrid Beam

The hybrid beam of the present invention is useful as a spar 105, rib,frame, or beam in high performance military or commercial aircraft. Thisinvention consists of two major components: A superplastically formedand diffusion bonded/brazed titanium I section 150 which forms the webof the spar or frame; and a thermoplastic or thermoset composite (sparor frame) cap 160 which is formed around the Titanium I section andconsolidated to it. The titanium web section can either be a straightsection or take on a sinusoidal wave pattern. For straight, untapered Isections the use of extrusion directly from titanium billet to near netshape is an attractive process. We can use three manufacturing processesto attach the caps to the titanium webs. In the first process, the capsare formed onto the titanium I section using an automated hot shoe toolthat runs down the spar, forming preconsolidated thermoplastic compositelaminate into a C section cap as shown in FIG. 2. This invention canalso be practiced by hand laying-up a thermoset tape laminate onto thetitanium I section as a internal lay-up mandrel tool to produce a hybridcomposite spar, frame, or rib as shown on FIG. 3. A thrid process formaking the beam involves forming a thermoset or thermoplastic cap onanother tool and either slip fitting or attaching the cap to the webfollowed by co-bonding the assembly in an autoclave or in our inductionheating workcell.

Although we can fabricate the cap from either thermoset or thermoplasticmaterials, we prefer thermoplastic caps because these materials can behot formed and molded onto the titanium I-section in a rapid, automatedprocess. Another advantage of thermoplastics is their inherent ductilitywhich allows us to produce thicker laminates of uniaxial lay-ups withoutthermal stress induced microcracking which would affect thermosetcomposites. The present invention addresses the fundamental problem withthe design of T or I section graphite composite spars and frames whosecap pulloff strength is limited by out-of-plane tensile stresses at thecap/web intersection. These out-of-plane stresses are between the gapfiller and the composite plies which wrap from the web into the cap andcause failure at this point in a pulloff test. In the conventionalall-composite sine wave spar approach, the out-of-plane loading limitsthe strength of the spar in pulloff to that of the resin which rangesfrom 18,000 psi for some thermoplastics to roughly 7000 psi forthermosets. These values are quite low relative to that of titanium(120,000 psi yield for Ti-6-4).

The beam also addresses the fundamental weakness of titanium spar orframe designs that have more than adequate cap pulloff strengths, butsuffer because titanium has a low specific stiffness relative tocomposite material and are therefore heavier than composite designs fora given stiffness, even though they are more resistant to damage.Another advantage of this hybrid beam design is the ability to use amajority of uniaxal plies of a thermoplastic composite material in thecaps. The shear (±45) plys that are present in traditional compositedesigns are no longer necessary because the thin titanium web sectionprovides the necessary shear transfer function. Laminates that contain ahigh percentage of uniaxial plies in their lay-up have up to a factor ofthree better stiffness in the longitudinal direction than a traditionalquasi-isotropic or fabric laminate used in traditional all-compositedesigns, resulting in a lighter, stiffer spar.

Our beam concept also solves the major difficulty of removing internaltooling from a closed box after welding the spars to the skin. Internaltooling is required when welding all composite spars and ribs to skinsto prevent the cap plys from deforming or delaminating when thetemperature is above the glass transition temperature. The I-sectiontitanium web of our beam acts to stiffen and support the cap during theinduction welding process, so that internal tooling is not required toweld the caps to thermoplastic skins.

The hybrid beam also provides lower total variation of the spar's heightas compared to conventional all-composite designs because the totalcomposite laminate thickness on the caps is much thinner than inall-composite designs which must include shear plys that transition intothe webs. In addition, the superplastically formed I-sections are moredimensionally accurate after fabrication than the corresponding portionof an all composite spar and can be finish machined for even betteraccuracy. A large variation in the spar height of about ±50 milsrequires extensive and costly shimming or machining of skin pad-ups tomake it fit with the mating skin during assembly. When dimensionalaccuracy is improved to less than ±8 mils (the maximum gap that can bepulled up in assembly of composites) the cost of wing assembly isreduced in traditional assembly processes and greatly improves thethermoplastic welding process.

Boeing made two previous attempts to produce hybrid composite(titanium/graphite composite) laminates. In the first, uniaxialthermoplastic composite material was interleaved with thin cold rolledbeta titanium to produce a hybrid titanium/graphite composite. Thisconcept is suitable for use as wing and fuselage skins as part of thinhoneycomb sandwich structures, but would not be useful for creatingspar, rib or frame I or T section stiffeners or beams. The secondattempt was to create a hybrid titanium/graphite composite spar byinserting a titanium T section into a conventional all composite spardesign in place of a rolled tape gap filler of a conventionalall-composite beam. In this second attempt, the pulloff load of the capwas increased by a factor of 7 from roughly 800 lbs/in to over 5600lbs/in but the arrangement of the plies, stiffness of the caps and themanufacturing processes to produce this design were the same as thoseused to fabricate conventional all composite spars. This first hybridspar design would not be capable of being welded to skins withoutinternal tooling and its shear webs would be more susceptible tohydraulic ram damage in a battle damage event than the beam of thepresent invention.

The T-section is preferably made by superplastic forming suitabletitanium sheet stock with two corresponding "C" elements bonded, welded,fused, brazed or fastened together. Normally the elements are diffusionbonded together after the SPF process is complete, although brazing maybe more reliable process as discussed in Boeing's U.S. Pat. No.5,420,400.

a. Forming the I-beam

Under certain conditions, some materials can be plastically deformedwithout rupture well beyond their normal limits, a property calledsuperplasticity. This property is exhibited by certain metals andalloys, within limited ranges of temperature and strain rate. Forexample, titanium and its alloys are superplastic in the temperaturerange from about 1450°-1850° F. (785°-1010° C.).

Superplastic forming (SPF) is a fabrication technique that relies onsuperplasticity. A typical SPF process involves placing one or moresheets of metal or plastic in a die, heating the sheets to an elevatedtemperature within the superplastic range, and superplastically formingthe sheet(s) at the SPF temperature. Generally, a differential formingpressure from a gas manifold is used to stretch the sheet(s) into thedesired shape against the die surface(s). This forming process can becalled blow molding insofar as it uses differential pressure to form thematerial. The differential pressure is selected to strain the materialat a strain rate that is within its superplastic range. The followingpatents are illustrative of SPF processes and equipment:

    ______________________________________                                        PATENT   TITLE             ISSUE DATE                                         ______________________________________                                        3,920,175                                                                              Method of SPF of Metals with                                                                    November 18, 1975                                           Concurrent Diffusion Bonding                                         3,927,817                                                                              Method for Making Metallic                                                                      December 23, 1975                                           Sandwich Structures                                                  3,605,477                                                                              Precision Forming of Titanium                                                                   September 29, 1971                                          Alloys and the Like by Use of                                                 Induction Heating                                                    4,141,484                                                                              Method of Making a Metallic                                                                     February 27, 1979                                           Structure by Combined Flow                                                    Forming and Bonding                                                  4,649,249                                                                              Induction Heating Platen for Hot                                                                March 10, 1987                                              Metal Working                                                        4,117,970                                                                              Method for Fabrication of                                                                       October 3, 1978                                             Honeycomb Structures                                                 5,024,369                                                                              Method to Produce June 18, 1991                                               Superplastically Formed                                                       Titanium Alloy Components                                            ______________________________________                                    

We incorporate these patents by reference.

One advantage of SPF is the forming of complex shapes from sheet metalwhile reducing the time and eliminating the waste of milling, producingconsiderable cost saving. In addition, the SPF process is generallyapplicable to single and multisheet fabrication. For multisheetfabrication, SPF is combined with joining processes, such as diffusionbonding, brazing or laser welding, to produce complex sandwichstructures. One advantage of the SPF process is lighter, lower costparts with fewer fasteners. A single part can replace the complexassembly currently required using conventional manufacturing operations.Common applications of SPF include the manufacture of parts foraircraft, missiles, and spacecraft.

In a typical prior art SPF process for titanium or its alloys, the sheetmetal is placed between dies, at least one of which has a contouredsurface corresponding to the shape of the product. The dies, are placedon platens which are heated, generally using embedded resistive heaters.The platens heat the dies to about 1650° F. (900° C.). Because thetitanium will readily oxidize at the elevated temperature, an inert gas,such as argon, surrounds the die and workpiece. The dies heat the sheetmetal to the temperature range where the sheet metal is superplastic.Then, under applied differential pressure, the sheet metal deformsagainst the contoured surface.

The platens and dies have a large thermal mass. They take considerabletime and energy to heat and are slow to change their temperature unlessdriven with high heat input or with active cooling. To save time andenergy, they must be held near the forming temperature throughout aproduction run (i.e., the production of a number of parts using the samedies). The raw sheet metal must be inserted onto the dies, and formedparts removed, at or near the elevated forming temperature. The hotparts must be handled carefully at this temperature to minimize bending.Within the SPF range, the SPF metals have the consistency of taffy, sobending can easily occur unless the operators take suitable precautions.

As described to some degree in U.S. Pat. No. 4,622,445 and in U.S. Pat.No. 5,410,132, we have discovered an improvement for an SPF processcoupling the use of cast ceramic dies with inductive heating. With ourinductively heated SPF press or workcell, we can heat preferentially thesheet metal workpiece with induction heating without heating the platensor dies significantly and can use the ceramic dies as an insulator tohold the induced heat in the part. We can stop the heating at any timeand can cool the part relatively quickly even before removing it fromthe die. We do not waste the energy otherwise required to heat the largethermal mass of the platens and dies. We do not force the pressoperators to work around the hot dies and platens. With our inductiveheating workcell, we also save time and energy when changing dies to setup to manufacture different parts because the dies and platen aresignificantly cooler than those in a conventional SPF press. We shortenthe operation to change dies by several hours. Therefore, the inductionheating process is an agile work tool for rapid prototyping or low rateproduction with improved efficiency and versatility.

The web is readily manufactured using conventional SPF processes andgenerally is shaped to include a transverse sinusoidal waveform shapefor increased strength.

We can form the beam from titanium sheet stock using our inductionheating workcell or in a conventional SPF press. We can diffusion bondor braze the complementary C sections in the same equipment and we mightcombine the forming and bonding (and even β-annealing, if desired)operations into a single cycle as described in our U.S. Pat. No.5,420,400 and U.S. patent application Ser. No. 08/452,216 entitledCombined Heating Cycles to Improve Efficiency in Inductive HeatingOperations, which we incorporate by reference.

b. Attaching the composite cap

The spar cap 160 is attached to the web section 150 with the methodsshown schematically in FIGS. 2 & 3. In FIG. 3, a prefabricated andpreconsolidated thermoplastic composite is heated and formed with a hotshoe 170 that passes slowly along the web from end to end. In FIG. 3,the cap 160 is prepared from prepreg that is laid up on the web section150 in the desired final configuration to surround the T. The lay-up isbagged with a conventional autoclave consolidation film material 180,and the pregreg is consolidated in an autoclave or other suitableprocess, using a caul plate 190 on the faying surface of the prepreg toobtain a smooth, finished surface and to assure equal pressure over theentire faying surface. Of course, other configurations might be used toattach the cap 160 to the web section 150. For example, the web mightinclude upright tips on the ends of the T to define a shallow channel inwhich the composite seats. Plies of the prepreg interfere with the lipof the tips. This shallow channel option is illustrated in FIG. 9.

The thermoplastic is typically PEI, PEEK, PEK, PEKK, or polyimidereinforced with glass, carbon, or graphite fibers. The web section istypically an SPF alloy of titanium. The T-section of the web canfunction as the induction susceptor in the welding operation, if the capis thin enough to melt at the faying surface when heated from within.Alternatively, a susceptor might be used at the bondline including, ifdesired, a reinforced susceptor fabric weave as the ply immediatelyadjacent the faying surface, as described in our U.S. patent applicationSer. No. 08/469,986, which we incorporate by reference, and asschematically illustrated in FIG. 8.

The hybrid beam is particularly suitable for use in thermoplasticwelding of aerospace wingbox or wing tip structure where ribs and sparsare welded to wingskins. We will next discuss our induction heatingequipment for performing thermoplastic welding and then will discuss thewelding process.

2. The Induction Heating Press

In FIG. 4, an induction heating workcell 10 includes tools or dies 20and 22 mounted within an upper 24 and a lower 26 strongback. Thestrongbacks are each threaded onto four threaded column supports orjackscrews 28 or they float free on the columns and are fixed with nuts.We can turn the jackscrews to move one strongback relative to the other.The strongbacks 24 and 26 provide a rigid, flat backing surface for theupper and lower dies 20 and 22 to prevent the dies from bending andcracking during manufacturing operations. Preferably, the strongbackshold the dies to a surface tolerance of ±0.003 inches per square foot ofthe forming surface. Such tolerances are desirable to achieve properpart tolerances. The strongbacks may be steel, aluminum, or any othermaterial capable of handling the loads present during forming orconsolidation, but we prefer materials that are nonmagnetic to avoid anydistortion to the magnetic field that our induction coils produce. Insome circumstances, the dies may be strong enough themselves thatstrongbacks are unnecessary. The strongbacks transfer pressure inputthrough the columns evenly to the dies.

The dies 20 and 22 are usually cast ceramic and are reinforced with aplurality of fiberglass rods 32 that are held with bolts 74 and thatextend both longitudinally and transversely in a grid through each die.Each die usually is framed with phonemic reinforcement 72 as well, tomaintain a compressive load on the die. Each die may be attached to itsstrongback by any suitable fastening device such as bolting or clamping.In the preferred embodiment, both dies are mounted on support plates 76which are held in place on the respective strongbacks through the use ofclamping bars 77. The clamping bars 77 extend around the periphery ofthe support plates 76 and are bolted to the respective strongbacksthrough the use of fasteners (not shown).

The dies should not be susceptible to inductive heating so that heatingis localized in the retort rather than distributed in the press as well.We prefer a ceramic that has a low coefficient of thermal expansion,good thermal shock resistance, and relatively high compression strength,such as a castable fused silica ceramic.

We embed portions of an induction coil 35 in the dies. In FIG. 4 we showfour separate induction segments that overlie the top and bottom of theworkpiece, but the number usually is higher, as shown in FIG. 6, and thesegments can surround the workpiece on the top, bottom, and all sides.Each segment is formed from a straight tubing section 36 that extendsalong the length of each die and a flexible coil connector 38 that joinsthe straight tubing sections 36 in the upper die 20 to the correspondingstraight tubing section in the lower die 22. Connectors 40 located atthe ends of the induction coil 35 connect the induction coil 35 to anexternal power source or coil driver 50 and to a circulation system forcooling fluid. While the tubes are shown as being circular incross-section, other shapes can be used, such as rectangular channels.

Cavities 42 and 44 in the respective dies hold tool inserts 46 and 48.The upper tool insert 46 in some applications has a contoured formingsurface 58 that has a shape corresponding to the desired shape of theouter mold line surface of the completed composite. The lower toolinsert determines the inner mold line. The tool inserts also should notbe susceptible to inductive heating, preferably being formed of acastable ceramic. In some cases, both the dies and the tool inserts canbe made from a matrix resin rather than from a ceramic. Using a resin,however, limits use of the tooling to low temperature operations, suchas forming or consolidating certain organic matrix composites. We preferceramic tooling which provides the greatest flexibility and versatilityfor the induction heating workcell.

While the forming surfaces can be an integral part of the dies, weprefer the separate die and tool insert configuration shown in FIG. 5because changing tool inserts to make different parts is easier andquicker (because they are significantly smaller) and the overall toolingcosts are reduced.

Each die surrounds and supports the respective tool insert and holds thestraight sections 36 of the induction coil in proper position inrelationship to the tool insert 46 or 48. In the preferred embodiment,the interior 70 of the dies is formed of a castable phonemic or ceramicand the exterior sides from precast composite phonemic resin blocks 72.In some applications, we prefer to reinforce the phonemic or ceramicwith chopped fibers or nonwoven or woven reinforcing mats.

FIG. 5 shows a retort 60 between tool inserts 46 and 48. The retort 60includes a workpiece and sandwiching susceptor facesheets. The retort isheated to a forming or consolidation temperature by energizing the coil35. In the case of a composite panel, when the panel reaches theconsolidation temperature at which the matrix resin flows, we apply gaspressure to the outer surfaces of the retort by pressure sources 52 and54. Pressure source 52 applies pressure to the upper surface of theretort 60 through a conduit 62 that passes through the upper die 20 andupper tool insert 46, while pressure source 54 applies a pressure to thelower surface of the retort 60 through a conduit 64 that passes throughthe lower die 22 and lower tool insert 48. The pressure applied to theretort 60 is maintained until the retort has formed to the contour ofthe forming surface 58 and the matrix resin has consolidated. Thepressure sources 52 and 54 generally apply a differential pressure tothe retort 60. We do not use a retort in the present invention.

An alternating oscillating electrical current in the induction coil 35produces a time varying magnetic field that heats the susceptor sheetsof the retort via eddy current heating. The frequency at which the coildriver 50 drives the coils 35 depends upon the nature of the retort 60.We power the coil with up to about 400 kW at frequencies of betweenabout 3-10 kHz. Current penetration of copper at 3 kHz is approximately0.06 inches (1.5 mm), while penetration at 10 kHz is approximately 0.03inches (0.75 mm).

The shape of the coil has a significant effect upon the magnetic fielduniformity. Field uniformity usually is important because temperatureuniformity induced in the retort is directly related to the uniformityof the magnetic field. Uniform heating insures that different portionsof the workpiece will reach the operating temperature at approximatelythe same time. Solenoid type induction coils like those we illustrateprovide a uniform magnetic field, and are preferred. Greater fielduniformity is produced in a retort that is located symmetrically alongthe centerline of the surrounding coil. Those of ordinary skill canestablish series/parallel induction coil combinations, variable turnspacing, and distances between the part and the induction coil bystandard electrical calculations to achieve the desired heating fromwhatever coil configuration is used.

The tool inserts and dies are usually substantially thermally insulatingand trap and contain heat within the retort. Since the dies and toolinserts are not inductively heated and act as insulators to maintainheat within the retort, the present invention requires far less energyto achieve the desired operating temperature than conventional autoclaveor resistive hot press methods where the metal tooling is a massive heatsink.

The operations using our workcell are faster than prior art operationsbecause we do not heat the large thermal mass of either the dies or toolinserts prior to the induction heating process. The susceptor is heated,the tool is not. Thus, the necessary processing temperature is achievedmore rapidly. In addition, the highly conductive materials provide rapidheat transfer to the workpiece. When the driver 50 is de-energized, thedies cool rapidly to a temperature at which we can remove the workpiecefrom the workcell, saving time and energy over conventional systems.Coolant flowing through the coil tubes functions as an active heatexchanger to transfer heat out of the workpiece, retort, and dies. Inaddition, the thermal cycle is not as limited by the heating and coolingcycle of the equipment and tools so we can tailor the thermocycle betterto the process for which we are using the induction heating workcell.

3. Thermoplastic Welding

As shown in FIGS. 6 & 7, we make several changes to our inductionheating workcell to adapt it to perform bondline thermoplastic welding.First, because the assemblies of primary interest are wingskin/sparcombinations and because the parts in these combinations areprefabricated so that the welding operation need only focus upon meltingthe thermoplastic while applying modest pressure to facilitate thefusion, we create a cavity within our dieset to contain thewingskin/spar combinations. The cavity is substantially a cube orsimilar rectangular solid with canted edges and has major surfaces(i.e., the top and bottom) complementary to the contour of the wingassembly. Our induction coils extend longitudinally in the samedirection as the spars and underlie the major and canted surfaces asshown in FIG. 6. The skins 100 and spars 105 are assembled in the centerof the cavity sandwiched between silicone rubber pressure pads 110 thatassure substantially uniform pressure distribution over the wingskinsurfaces irrespective of surface features or irregularities. A susceptortape 115 usually is positioned along the bondline between the wingskin100 and the spar caps. By a "susceptor tape" we mean a metal ribbonembedded in thermoplastic resin or a structural susceptor as describedin U.S. patent application Ser. No. 08/471,625 having the resin-embeddedribbon sandwiched with alternating plies of thermoplastic film and fiberreinforcement to alleviate residual tensile strain in the weld and tosimplify gap filling while ensuring a resin rich, quality weld. Themetal ribbon may be copper, a cobalt alloy, nickel-iron alloys, or anyother suitable "smart" susceptor from the alternatives discussed in U.S.patent application Ser. No. 08/469,604. The susceptor might be narrowmetal strips about 0.10-0.20 in wide held in side-by-side array with thethermoplastic resin or woven with carbon fibers or other reinforcement.The induction coil of our induction heating workcell induces eddycurrents that run longitudinally. Therefore, the susceptor should have alower longitudinal impedance to promote longitudinal current flow. Wemight use a modified, selvaged susceptor (see U.S. patent applicationSer. No. 08/314,027) having solid copper bands alternating with meshsections with the solid bands in the bondline rather than fallingoutside it, since they are the primary current carriers.

A "susceptor tape," however, still suffers from a relatively low bondstrength because the metal susceptor is asked to function as theequivalent of a reinforcing fiber. The matrix resin, however, does notwet with the metal as well as it does with the reinforcing fibers andthe metal does not have the strength commonly available with the fibers.Therefore, a reinforced susceptor promises improved bond strength.

The need for a susceptor in the bondline poses many obstacles to thepreparation of quality parts. The metal which is used because of itshigh susceptibility differs markedly in physical properties from theresin or fiber reinforcement so dealing with it becomes a significantissue. The reinforced susceptor (FIG. 8) overcomes problems withconventional susceptors by including the delicate metal foils 200(0.10-0.20 in wide×0.005-0.010 in thick; preferably 0.10×0.007 in) intandem with the warp fibers 205 of the woven reinforcement fabric. Thefoil is always on the remote side of the fabric because it is betweenthe warp thread and the weave threads 210. This arrangement holds thefoils in place longitudinally in the fabric in electrical isolation fromeach other yet substantially covering the entire width of the weldsurface while still having adequate space for the flow and fusion of thethermoplastic resin. Furthermore, in the bondline, the resin cancontact, wet, and bond with the reinforcing fiber rather than beingpresented with the resinphilic metal of the conventional systems. Therewill be a resin-fiber interface with only short runs of a resin-metalinterface. The short runs are the length of the diameter of two weavefibers plus the spatial gap between the weave fibers, which is quitesmall. Thus, the metal is shielded within the fabric and a better bondresults. In this woven arrangement to foil can assume readily thecontour of the reinforcement. Finally, the arrangement permits efficientheat transfer from the foil to the resin in the spatial region where thebond will form. The reinforced susceptor might be an analog of thestructural, selvaged, or tailored susceptors of one other application(i.e. a tape encased in resin and placed along the bondline) or may befabricated as part of the facing flies of the prefabricated compositesthat abut along the bondline.

As shown in FIG. 7, the susceptors at the bondlines for the top andbottom are connected together into a loop circuit with jumpers 115 atthe ends of the spars 105. The jumpers 115 allow the current which themagnetic field induces to flow around the assembly to generate heat inthe bondlines.

With the beam of the present invention, the web's spar cap ("flange")functions as an embedded susceptor so it may be unnecessary to use anadditional susceptor at the bondline. The flange is oriented properly inthe assembly to heat under the time varying magnetic field from theinduction coil (moving or fixed). The flange, accordingly can heat thecomposite cap and with its melting, the cap can create the desiredfusion bond or weld.

With the wingskin/spar combination assembled on the pressure pads in thecavity, we close the dies and energize the coil 35 using a frequency ofabout 3-10 kHz to produce about 400 kw. This energy produces anoscillating magnetic field around the assembly (which preferably isaligned with the central axis of the coil) that rapidly heats thesusceptors to the desired welding temperature. If we use a bondlinesusceptor, we prefer to use a "smart" susceptor made from a nickel-ironalloy as discussed in U.S. patent application Ser. No. 08/469,604entitled Method for Achieving Thermal Uniformity in Induction Processingof Organic Matrix Composites or Metals, which will assure that we do notoverheat the bondline as well as assuring a substantially uniformtemperature in the bondline during the fusion period when thethermoplastic resin is melted. As shown in FIG. 6, we simultaneouslymake the six welds (one weld on each spar cap of the 3 spars), whichgreatly reduces processing time. The welding process is quite fasttaking about 25-30 minutes including heating to the melt, holding thetemperature during the weld fusion, and cooling. Throughout the process,we maintain a pressure of about 30-100 psi along the bondline. Theweight of the assembly may make the pressure slightly higher on thebottom than the top but this pressure difference should be insignificantto the quality of the weld and the performance of the completed part.

The integrity of the weld is critical to the performance of thecompleted, welded structure. The quality of the weld is related to thetemperature along the bondline and good welds require control of thetemperature within a relatively narrow range during the welding. Weparticularly want to avoid overheating, so a "smart" susceptor made froma Co, Ne, or Fe alloy with a Curie temperature slightly above themelting temperature of the resin will help ensure that we producequality welds. Furthermore, an alloy like Invar 42 (42% Ni-58% Fe) has acoefficient of thermal expansion (CTE) comparable to the resin compositeso that embedding the susceptor into the completed part will not have asdramatic an impact if the susceptor is such an alloy rather than copperor another metal where the CTE mismatch between the resin and susceptoris larger. We might use an Invar cladding (especially age hardened Invar42) on the titanium spar cap to obtain the "smart" susceptorperformance, but worry about mixed metal interfaces in the product.Alternatively, the Invar can be brazed to the spar cap as shown in FIG.10. The "smart" susceptor includes sharp, barbed prongs 250 (FIGS. 10,11, & 12) which extend in the Z-direction out of the plane of thesusceptor and bondline. These prongs embed in the composite laminateduring the welding process to increase significantly the pulloffstrength of the weld. The "smart" susceptor is described in our U.S.patent application Ser. No. 08/486,560 entitled Barbed Susceptor forImproving Pulloff Strength of Welded Thermoplastic Composites, which weincorporate by reference. The "smart" susceptor might also be made frommartensitic 400 series stainless steel or INCO 909 iron-nickel alloy.The prongs 250 generally extend upwardly about 0.020-0.030 in. They arebarbed by making slanted laser cuts along their shafts to allow theprongs to penetrate easily but to pull out with difficulty.

The present invention is applicable to all types of organic matrixcomposites including both thermosetting and the thermoplastic compositessuch as epoxies, polyimides, PEEK, PEK, PEKK, PES, or the like. It isespecially suited, however, for consolidation or forming of resins thathave low volatiles content and that are nonreactive (i.e., the truethermoplastics like PEEK or ULTEM).

The surface of an aircraft wing skin must be maintained to a closetolerance to achieve an efficient aerodynamic surface. The tolerances ofthe inner mold line surface of the wing skin must also be maintained ata close tolerance at least in a buildup area where the wing skin will bejoined to a spar to ensure that the wing skin and spar can be preciselyjoined. It is not critical, however, to control the inner mold linesurface in areas where the wing skin is not attached to otherstructures. The composite panel has additional plies to define thebuildup areas. The additional reinforce the composite panel in theseareas which is necessary where a spar will be attached, and provide aconvenient way to match the skin and spar to produce the desired outerwing configuration even if the spars are imprecise in their dimensions.We can fabricate built up areas at the faying surfaces to provide theprecision fit, in which we can eliminate shims.

In thermoplastic welding, the susceptor may be in sheet, mesh, expanded,milled, selvaged or other suitable form and should be structured fromthe optimum conductivity longitudinally and transversely needed toobtain controlled, reliable, and reproducible heating. Geometry andstructure are closely related to the type of induction head used, asthose of ordinary skill will understand. Therefore, the spar caps mightinclude etched, stamped, or machined apertures in a predeterminedpattern to produce the desired heating pattern.

If the spar caps of the metal web have a pattern of openings, severalother advantages result. First, fusion bonds between the plies overlyingthe cap and those that bend around can be formed upon curing. Second,Z-pin reinforcement of the nature described in U.S. Pat. No. 4,808,461can be added to further assist in holding the composite on the metal.During welding, these Z-pins might also help to strengthen the weld bondthat forms at the interface between the beam and the skin.

While we have described preferred embodiments, those skilled in the artwill readily recognize alterations, variations, and modifications whichmight be made without departing from the inventive concept. Therefore,interpret the claims liberally with the support of the full range ofequivalents known to those of ordinary skill based upon thisdescription. The examples illustrate the invention and not intended tolimit it. Accordingly, limit the claims only as necessary in view of thepertinent prior art.

We claim:
 1. A welded structure, comprising:(a) at least oneprefabricated, organic matrix composite having a fiber-reinforcedorganic matrix resin; (b) a hybrid metal webbed, composite capped beamassembled with the composite to define at least one bondline between thecomposite and the beam cap, the beam cap being a fiber-reinforcedorganic matrix resin; and (c) a weld of organic matrix resin joining thecomposite and beam cap along the bondline.
 2. The welded structure ofclaim 1 wherein the beam web includes a flange to capture the beam cap.3. The welded structure of claim 2 wherein the flange is susceptible toheating by induction when exposed to a time-varying magnetic field andis capable of heating the bondline sufficiently to form the weld.
 4. Thewelded structure of claim 1 wherein the web includes sinusoidalcurvature.
 5. The welded structure of claim 1 further comprising asusceptor in the bondline to facilitate heating of the weld.
 6. Thewelded structure of claim 5 wherein the susceptor includes Z-plane barbsto improve pulloff strength in the weld.
 7. The welded structure ofclaim 6 wherein the susceptor is made from a material having a Curietemperature of less than or equal to about 700° F.
 8. The weldedstructure of claim 1 further comprising fiber reinforcement in the weldsufficient to alleviate residual tensile strain.
 9. The welded structureof claim 1 wherein the matrix resin in the composite, beam cap, and weldare the same thermoplastic resin.
 10. A welded aerospace structure usinga hybrid metal webbed composite beam obtainable by:(a) assembling apreform to define a bondline, the preform having a prefabricated organicmatrix resin composite adjacent a hybrid metal webbed composite cappedbeam, the beam having a beam web and at least one beam cap, the beam webincluding a flange for capturing the beam cap and being aligned with thebondline; and (b) inducing a current in the flange to heat the bondlinesufficiently to join the composite to the beam cap through a resin weld.11. The structure of claim 10 wherein the flange includes a plurality ofprongs in a Z-direction out of the plane of the flange and bondline sothat the weld has improved pulloff strength.
 12. The structure of claim10 wherein the flange is clad with a susceptible material that has aCurie temperature slightly above the melting temperature of the resin ofthe organic matrix resin composite.
 13. The structure of claim 10wherein the resin of the organic matrix resin composite is polyimide,PEEK, PEK, PEKK, or PES.
 14. The structure of claim 10 wherein thecurrent is induced by using an induction coil that moves along thebondline.
 15. The structure of claim 10 further comprising:reinforcingthe resin weld by including at least one reinforcing fiber ply in theweld to reduce residual tensile strain.
 16. The structure of claim 10wherein the flange is titanium or titanium alloy.
 17. The structure ofclaim 10 further comprising:reinforcing the resin weld with Z-pinreinforcement to assist holding the composite on the flange.